The present invention relates generally to semiconductor laser controllers, and more particularly to a semiconductor laser controller which controls an optical output of a semiconductor laser used as a light source in a laser printer, optical disk storage, digital copier, light communication apparatus, etc.
Since a semiconductor laser can be made very small and achieve a direct modulation at a high speed, it has been recently widely used as a light source for optical disk storages, laser printer, etc. However, the optical output of semiconductor lasers is very sensitive to changes in temperature, and there exists a problem even when the driving current flowing therethrough is made constant, hence it is difficult to stably set the optical output thereof at a desired level. Accordingly, various Automatic Power Controllers have been developed to cope with the above problem.
The automatic power controllers are roughly classified into types which use one of the following three methods:
1. According to the first method, optical output of a semiconductor laser is always controlled by an optical-electronic negative feedback loop. A light-receiving element receives the optical output and generates light current proportional thereto. The light current is then compared with power-setting current which determines a desired optical output of the semiconductor laser. The (driving) forward current is adjusted so that the light current can be equal to the power-setting current. Thus, according to this method, the optical output is controlled stably (i.e., with high reliability) since it is always thus controlled.
2. According to the second method, forward current of the semiconductor laser is controlled, only during a power-adjusting period, so that the light current can correspond to the power-setting current. During a non-power-adjusting period, the forward current just adjusted during the previous power-adjusting period flows through the semiconductor laser. Thus, according to this method, the optical output can be controlled quickly (i.e., with a high response) since it is controlled only during the power-adjusting period.
3. The third method improves upon the second method in that it includes the temperature of the semiconductor laser among the controlled factors. The temperature of the semiconductor is measured, and the forward current and/or the temperature are then adjusted.
However, the above conventional methods have the following disadvantages. The first method has a disadvantage in having a low control speed, since the optical output is always controlled. In addition, a gain of the feedback loop cannot be compensated. Hereupon, a step response of the optical output of the semiconductor laser is approximated as follows: EQU P.sub.out =P.sub.0 [1-exp(-2.pi.f.sub.0 t]
where P.sub.out represents the optical output of the semiconductor laser, P.sub.0 represents a desired optical output of the semiconductor laser, f.sub.0 represents a gain crossover frequency in a case where the optical-electronic negative loop is open, and t represents time. Generally, as to the control speed, a total light amount (.intg.P.sub.out) of the semiconductor laser, defined below, must converge into a predetermined value within a settling time .tau..sub.0, just after a change in the optical output: ##EQU1##
If an attempt is made to improve upon the control speed of the optical output of the semiconductor laser, by adjusting the gain crossover frequency f.sub.0 in a case where .tau..sub.0 is made to be 50 ns and an allowable error range is made to be 0.4%, f.sub.0 must be higher than 800 MHz, which is extremely difficult.
On the other hand, the second and third methods have a disadvantage in having a low reliability since the optical output is not always controlled. As a result, the optical output is sensitive to disturbances, such as a DO loop characteristic, or return light which is often seen in an optical disk storage. The DO loop characteristic changes the optical output by several percents. The DO loop characteristic can be prevented, to some degree, from influencing the optical output by adjusting the forward current based on a heat time constant of the semiconductor laser, however, it cannot be completely prevented, in particular, in a light source using a plurality of semiconductor lasers, since each heat time constant is different for each semiconductor laser and the environment in corresponding semiconductor laser.
Incidentally, a semiconductor laser controller was disclosed in Japanese Laid-Open Patent Application No. 2-205086 designed to eliminate the above disadvantages. The semiconductor laser controller uses an optical-electronic negative loop which calculates a difference between the light current and the power-setting current, and converting means for converting the power-setting current into the forward current. The semiconductor laser is controlled by adding or subtracting a control signal, generated by the optical electronic negative loop to or from the forward current generated by the converter means. However, this disclosed semiconductor laser controller also has disadvantages in having low speed, low reliability, and low resolution.